The decision to measure exosomal miRNA from serum is supported by a study of miRNA in 12 human body fluids. Weber and colleagues (2010) determined that the median total concentration of RNA in plasma (308 ug/L) was greater than that of cerebrospinal fluid (111 ug/L). Furthermore, the authors found the number of detectable miRNAs in plasma (349) to be greater than cerebrospinal fluid (212). The miRNA content of plasma and cerebrospinal fluid has some intriguing similarities. Of the twenty most abundant miRNAs found in cerebrospinal fluid and plasma, 9 overlap. We have previously examined the expression of exosomal miRNA in the serum of human alcoholic subjects. Intriguingly, we characterized the expression levels of brain-specific miRNAs in serum samples from over 30% of those tested. This finding demonstrates that exosomal miRNAs from the central nervous system can be detected and quantified in human blood samples.